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Research paper : A challenge to the low-cost production of highly functional optical elements (J. Nishii)−25−Synthesiology - English edition Vol.1 No.1 (2008) advancement of theory and design. This process has been continued to this day.2.1 Barrier of costLet me direct attention to the field called “resonance and sub-wavelength”, for which the research was started around 1990. Computer simulations demonstrated that the highly functional elements could be actualized if the periodic structures comparable to wavelength of visible light or less could be fabricated precisely. Later, some basic studies on the fabrication of prototypes were carried out actively, alongside the progress of semiconductor microfabrication technology such as silicon[1].The wavelength discussed in this paper is from visible to near-infrared region (wavelength about 400 nm to 2000 nm). If the period of structure is 2n times (n = 0, 1, 2, 3…) of the wavelength of incident light, the optical diffraction will occur. Such structures will also cause strong reflection and light trapping due to resonance of light within the periodic structure. When the period of structure decreases, diffraction and resonance do not occur, and the refractive index of such periodic structure can be considered as the average of air and material. This is the principle of optical devices classified in the “resonance and sub-wavelength” domain.Although several optical elements have been realized in resonance and sub-wavelength domain, their applications were limited. Namely they were not installed widely in commercial devices such as home electronics products, because the industrial arena required an extremely low production cost and a large production scale of several million elements or more per month.2.2 Barrier of functionThe molding process and injection process invented in 1980 enabled the productions of aspheric lenses and diffraction gratings, which used to be difficult to fabricate at low cost with conventional grinding and polishing methods. The fabrication technology of precise molds accelerated the mass-production of several optical elements with various forms including aspheric lenses. For example, the glass lenses fabricated by the molding are used in almost 100 % of zoom optics for digital still cameras. Hundreds of millions of lenses are produced each year in Japan and neighboring countries. However, new demands arose for comprehensive suppression of ghost, spherical aberration, and color aberration, in addition to high-resolution, downsizing, and weight reduction. On the other hand, since the blue-ray with 405 nm wavelength is used in next-generation optical disc drive, which is called Blu-ray, the new optical element compatible with 3 wavelengths including conventional CD (wavelength 785 nm) and DVD (wavelength 655 nm) is required. To fulfill the needs of such functional elements, new technology is necessary to incorporate the several functions such as refraction and diffraction, structural birefringence without wavelength dependence, and antireflection with little dependence both on wavelength and incident angle, into conventional optical elements such as lens.2.3 Objective of this researchConventional imprinting technology can be used only in resin because there is an upper limit in molding temperature. There was no report on the application of the imprinting process to glass that requires high temperature of several hundred degrees. Also, glass-molding process is used for the production of optical elements with flat surface such as lenses. Eventually, there was almost no approach to forming structures smaller than wavelength of visible light. Thus the objective of this study is to develop new fabrication technology of resonance and sub-wavelength optical elements, which have been fabricated using microfabrication technologies so far, through the development of glass imprinting process by combining the imprinting process for resin material used in the academia, and the glass molding process used in industry.3 Scenario to achieve objectiveThe size required for the resonance and sub-wavelength optical elements is in the range of tens of nm to several µm. It is also necessary to fabricate microstructures on a large surface in a shorter time possible. It should be out of the range covered by the methods such as lithography and etching, laser process, or mechanical process, which are currently used in industry. Such microstructures should be formed mostly on the surface of lenses, prisms, or window materials. Therefore, it is advantageous to transfer such structures to the surface of the optical elements using the principle of imprinting process, if the thermally durable molds can be fabricated. The concepts of molding process and imprinting process are shown in Figure 2. Imprinting process was first reported by Chou et al. of Princeton University in the United States[2, 3]. It is a method where the mold with nano-structure is pressed against the resin, and the structure is transferred using ultraviolet light or heat[4]. So far, the products realized using the imprinting process are mainly based on the resin materials such as light glass molding methodimprinting methodFig. 2 Schematic diagram of glass molding method and imprinting methodHeat resistant, ultra-hard material is used for glass molding, while glass or silicon is used for imprinting mold.

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